Recently, an organic EL display using an organic EL device as a light emitting device has been studied and developed. In the organic EL display like this, an active-matrix (AM) organic EL display in which a driving circuit is provided in each pixel is generally used to extend the life span of the organic EL device and achieve high-quality image. The relevant driving circuit is constituted by a thin film transistor (TFT) formed on a substrate such as glass, plastic or the like. In the organic EL display, the substrate and the driving circuit portion are together called a back plane.
As the TFT of the back plane for the organic EL display, amorphous silicon (called a-Si hereinafter), polycrystal silicon (called p-Si hereinafter), or the like have been studied. In addition, a TFT in which an amorphous oxide semiconductor (called an AOS hereinafter) is used as its channel layer has newly been proposed recently. Here, for example, amorphous In (indium)-Ga (gallium)-Zn (zinc)-O (oxide) (called a-IGZO hereinafter) is used as the material of the AOS. Besides, for example, amorphous Zn (zinc)-In (indium)-O (oxide) (called a-ZIO hereinafter) is used as the material of the AOS. It is conceivable that the TFT in which the AOS is used as its channel layer has mobility which is ten times or more as much as that of an a-Si TFT and also has high uniformity which is caused by amorphousness. Therefore, the TFT in which the AOS is used as its channel layer is promising as the TFT of the back plane for the display. The TFT in which the AOS is used as its channel layer is disclosed in, for example, “Nomura, et al., Room-Temperature Fabrication of Transparent Flexible Thin Film Transistors using Amorphous Oxide Semiconductors, Nature, vol. 432, pp. 488-492 (2004)” and “Yabuta, et al., High-Mobility Thin-Film Transistor with Amorphous InGaZnO4 Channel Fabricated by Room Temperature RF-magnetron Sputtering, Appl. Phys. Lett. (APL), 89, 112123 (2006)”.
In any case, there are several problems in case of achieving high-quality display by an active matrix (AM) organic EL display. More specifically, (1) a voltage-luminance characteristic of an organic EL device changes over time, (2) a characteristic of a TFT being the constituent element of a driving circuit varies from others, and (3) the characteristic of the TFT changes due to an electrical stress.
Here, in a case where an AOS-TFT is used for the driving circuit, the above problems (1) and (2) can be improved because uniformity of the AOS-TFTs is high and a driving circuit for controlling the currents supplied from the AOS-TFT to the organic EL device is employed.
On the other hand, since the characteristic of the AOS-TFT changes due to the electrical stress, the above problem (3) still remains.